Amarin Pharmaceuticals has been a biotech success story. Over the past two years the share price has tripled as the tiny company has managed to get approval from the Food and Drug Administration for Vascepa, a fish-oil pill that in many ways seems set to take market share from a blockbuster competitor, GlaxoSmithKline’s Lovaza. There are even reports that AstraZeneca and Teva Pharmaceuticals have considered buying it. But there may be rough seas ahead. Just a few years ago, Vascepa seemed like a marketer’s dream. Both it and Lovaza are approved to lower very high triglycerides, particles of fat in the blood that are thought to play a role in heart disease and pancreatitis. They do this because they contain long-chain omega-3 fatty acids, special “good” fats that are scarce in the human diet. Lovaza, which generated $240 million in sales for Glaxo last quarter, also raises low-density lipoprotein, the “bad cholesterol.” Vascepa doesn’t. There are other reasons that Vascepa might be better, but that alone is a pretty great marketing message. But now scientific publications, comments from doctors, and prescription trends all hint that the market for prescription fish oil in the U.S. may be shifting dramatically against fish oil and Lovaza use is going down. Investors have been focused on whether Vascepa has strong enough patent protection. But what if the real problem is the state of the fish oil market?